The present invention relates to a power supply circuit. Particularly, the present invention relates to a power supply circuit in which cross-regulation problems (cross-regulation) are improved.
In a switching power supply device, an output voltage is monitored by comparing it with a reference voltage in an error amplifier. Thus, output from a power supply circuit is controlled so as to consistently output a correct voltage. When the switching power supply device has a single output, there is no problem monitoring the output voltage. However, when the switching power supply device has a plurality of outputs, there is a problem. That is, when one output is monitored for stabilizing an output voltage, an output voltage of another output may not be stabilized. Specifically, when a load variation occurs in an output in which the output stabilization is monitored, it affects another output that commonly uses a primary winding (coil). This phenomenon is referred to as cross-regulation. Cross-regulation is one well-known problem of a multiple-output power supply. The cross-regulation is determined based on an actual output voltage regulation with respect to a reference voltage of an output terminal.
A switching power supply device that has two or more output terminals is widely used. However, it is known that voltage regulation may deteriorate when there is a big difference between powers that are output from the output terminals. FIG. 1 shows a conventional two-output flyback power supply device (circuit) 1. The power supply device 1 is configured with a primary coil (winding) Np, a first output coil (a secondary coil) Ns1, a second output coil (the secondary coil) Ns2, a switching element Q1, a control circuit Ic (not shown), and rectifying devices (diodes) D1 and D2. Specifically, the first output coil Ns1 and the second output coil Ns2 are coupled to each other. The switching element Q1 is connected in series to the primary coil Np. The control circuit Ic controls the switching element Q1. The rectifying device (the diode) D1 supplies electric power to a first output terminal Vo1. Further, the rectifying device (the diode) D2 supplies electric power to a second output terminal Vo2.
In the power supply device 1 explained above, an output voltage of the first output terminal Vo1 is typically monitored (feedback control) and a duty of the switching element (transistor) Q1 is adjusted so as to stabilize the output voltage value. An output voltage of the second output terminal Vo2 is not usually monitored (feedback control). As a result, when there is a big difference between the output voltages (electric power) of the output terminals, there is a possibility that a voltage regulation of the output terminal (here, the second output terminal Vo2) for which the feedback control is not performed increases.
In a circuit shown in FIG. 1, the output voltage of the first output terminal Vo1 corresponds to 10V (with the feedback control). Further, a load for the first output terminal Vo1 corresponds to 0.1 A-20 A. The output voltage of the second output terminal Vo2 corresponds to 5V (without the feedback control). Further, a load for the second output terminal Vo2 corresponds to 5 A (a fixed value). In this case, FIG. 2 shows the output voltages of the first and second terminals Vo1 and Vo2, and a voltage variation of the second terminal Vo2. As shown in FIG. 2, the second output terminal Vo2 is very unstable and the output voltage is significantly changed in a range of 3V to 6V.
In order to resolve the deterioration of the voltage regulation explained above and to stabilize output voltage, a technique has been adopted in which the load is artificially increased by, for example, a dummy resistor. However, in this case, a deterioration of power conversion efficiency cannot be avoided.